A window structure InGaAsP avalanche photodiode (a.p.d.) has been fabricated by using liquid-phase epitaxy on (111B) InP. Multiplication characteristics show that the impact ionisation rate for electrons is larger than that for holes (α>β). The ionisation ratio α/β was estimated to be 3–4. A low-noise a.p.d. will be realised by a structure with electron-initiated multiplication.